Apparatus for pressure sensing

ABSTRACT

The present invention provides an apparatus for distributed pressure sensing. The apparatus comprises a series of Bragg gratings, a light guide incorporating the series of Bragg gratings and a plurality of moveable wall portions. The moveable wall portions are coupled to respective Bragg gratings so that the movement of one of the moveable wall portion causes a force on the respective Bragg grating resulting in a change in strain of the respective Bragg grating. The apparatus also comprises at least one rigid member that is attached at attachment regions between which a sensing region of at least one Bragg grating is defined. The rigid member is arranged so that a strain in the sensing region is not directly influenced by a change in strain of the light guide outside the sensing region.

FIELD OF THE INVENTION

The present invention broadly relates to an apparatus for pressuresensing.

BACKGROUND OF THE INVENTION

The human body has many regions in which pressure differences causematter to move. For example, the human heart pumps blood through thebody. Muscles around the alimentary, canal apply a pressure to thechannel which moves food from the mouth into the stomach. Further, apressure increase in a portion of the body may be caused by a chemicalreaction such as the development of a gas in an enclosed body cavity.

Monitoring pressures in the human body can provide important informationabout the function of the human body and can be used to detect disordersand diseases or can be used to control a recovery from a disease.

For example, dysphagia, which is a disorder that causes difficulty inswallowing, typically affects infants and elderly people and isespecially prevalent in post-stroke patients. It is difficult todiagnose this disease and diagnostic tools are often very uncomfortablefor the patient.

A multi-bore catheter tube is commonly used for diagnosis of thisdisorder and the multi-bore catheter is inserted into the oesophagus.The exit ports of the bores of the catheter are positioned at differentlocations along the catheter and a steady flow of water exits througheach port. Measurement of the hydraulic water pressure at an input ofeach bore gives an indication of the pressure distribution in theoesophagus and therefore can be used to diagnose the disorder.

Another method of in-vivo pressure measurement involves usage of aseries of piezoelectric or electro-mechanical devices. Such devicestypically are expensive and require a relatively large number ofelectrical wires to be contained in a catheter which consequently is ofrelatively large thickness. The device is inserted through the nose ofthe patient and its relatively large diameter results in discomfort forthe patient.

Recently optical pressure measurement devices became popular in which anexternal pressure change effects a change in light interferenceconditions which can be detected. Such an optical device may comprise afibre Bragg grating which has an optical response that depends on astrain of the Bragg grating.

The present invention provides an improved technological solution.

SUMMARY OF THE INVENTION

The present invention provides in a first aspect an apparatus fordistributed pressure sensing, the apparatus comprising:

a series of Bragg gratings,

a light guide incorporating the series of Bragg gratings,

a plurality of a moveable wall portion having opposite first and secondsides, each moveable wall portion being positioned so that a change inpressure at one of the sides relative to a pressure at the other sidewill move the moveable wall portion, the moveable wall portions beingcoupled to respective Bragg gratings so that the movement of one of themoveable wall portion causes a force on the respective Bragg gratingresulting in a change in strain of the respective Bragg grating and

at least one rigid member that is rigidly attached to the light guide atattachment regions,

at least one sensing region extending from one attachment region to anadjacent attachment region,

wherein the rigid member is arranged so that a strain in the sensingregion is not directly influenced by a change in strain of the lightguide outside the sensing region.

The change in strain of each Bragg grating causes a change in an opticalresponse of the Bragg grating to light that is in use guided to eachBragg grating so that the changes in external pressure at the locationof each Bragg gratings can be detected. Because the apparatus isarranged so that a strain of any one of the sensing regions is notdirectly influenced by a change in strain of the light guide outsidethat sensing region, it is possible to measure the pressure at theposition of a plurality of sensing region largely independent from eachother and thereby measure a pressure distribution. This also allows tomeasure pressure at a plurality of sensing regions independently of eachother in an environment that inherently applies an axial strain to themeasuring device, such as exists in the human oesophagus.

The apparatus typically comprises a plurality of enclosures, eachenclosure enclosing a space and comprising a respective rigid member,which may be a rigid wall portion or casing of the enclosure, and arespective moveable wall portion coupled to a respective Bragg grating.Each enclosure typically is attached to the light guide at attachmentregions so that a respective sensing region of a respective Bragggrating is positioned between attachment regions and the enclosureprevents that a strain in that sensing regions is directly influenced bya change in strain of the light guide outside that sensing region, forexample by a change in strain at another sensing region.

The apparatus may comprise an external catheter that may be arranged forinsertion into a human body. Further, the apparatus may comprise aportion comprising an X-ray opaque material which enables imaging theposition of the apparatus in the human body. For example, the apparatusmay be arranged for positioning in the oesophagus of the human body fordistributed pressure measurement in the oesophagus. With theabove-described apparatus it is possible to measure a distributedpressure in the oesophagus in a manner such that a swallowing action orfood travelling past a selected region does not significantly influencea pressure reading at another sensing region by an axial strainoriginating from the selected sensing region.

Each Bragg grating of the series typically is arranged to give adifferent optical response so that light reflected from each Bragggrating is wavelength division multiplexed. As each Bragg grating givesa different response, it is possible to associate a particular pressurechange with a respective position.

In a variation of this embodiment at least some of the Bragg gratingsare substantially identical and give the same response if the strainconditions are the same. Using time domain reflectometry techniques, theposition of a particular Bragg grating may be estimated from a time atwhich an optical response is received.

The light guide may comprise one optical fibre which may compriseportions that are spliced together. The optical light guide typically isattached at the attachment regions, but typically is flexible at regionsbetween adjacent enclosures so that the apparatus is articulated.

The force on each Bragg grating typically is a force on a side portionof each Bragg grating. The apparatus typically is arranged so that theforces on the side portions are applied from one side of each Bragggrating at the sensing region. The apparatus may be arranged so that theforces are applied in any transversal or non-axial direction of theBragg grating, but the apparatus typically is arranged so that theforces are applied in a direction that is substantially perpendicular toan axis of respective Bragg gratings.

For example, each wall moveable portion may be a diaphragm.

In one specific embodiment the apparatus has a normal operatingtemperature and pressure range at which the Bragg gratings are distortedinto respective enclosed spaces. The apparatus may be arranged so that atemperature related change in optical response of each Bragg grating isreduced by a change in the forces on the Bragg gratings caused by atemperature related change in the respective enclosed volume.

The apparatus may be arranged so that a temperature related change in aproperty of the moveable wall portion, which typically is positionedadjacent a respective Bragg grating, reduces the temperature relatedchange in the optical response of the Bragg gratings. In this case theapparatus has the particular advantage that the moveable wall portionhas a dual function, namely reducing the temperature related change inthe optical period of the Bragg grating and causing a force on the Bragggrating in response to an external pressure change. The dual functionfacilitates a compact design of the enclosures and the pitch of theapparatus, or the distance between adjacent sensing regions, may berelatively small.

The apparatus may be used for pressure measurements in any environment,including for example in-vivo-environments, laboratories and windtunnels.

The Bragg gratings typically are positioned on respective moveable wallportions and outside the enclosures. Alternatively, the Bragg gratingsmay be positioned within the moveable wall portions or on the moveablewall portions and inside respective enclosures.

Each moveable wall portion may be positioned opposite a non-moveablewall portion of a respective enclosure. In this case the apparatus issuitable for sensing pressure changes on one side of the apparatus.Alternatively, each moveable wall portion may surround a portion of theenclosed volume of a respective enclosure. In this case each Bragggrating typically also surrounds at least a portion of a respectiveenclosed volume.

In another specific embodiment each moveable wall portion and each Bragggrating surrounds an entire respective enclosed volume and the apparatusis arranged so that pressure changes can be sensed in a region thatradially surrounds the apparatus.

Each enclosure typically is filled with a compressible fluid such asair.

The apparatus may be arranged so that the optical response from eachBragg grating can be detected by detecting light that is reflected backfrom the Bragg gratings. In this case the light guide typically isarranged so that the light is guided to and from the Bragg gratings bythe same optical fibre portion.

The apparatus may also be arranged so that the optical responses fromthe Bragg gratings can be detected by detecting light that istransmitted through the Bragg gratings. In this case the light guidetypically comprises at least one optical fibre for guiding the light tothe Bragg gratings and at least one other optical fibre for guiding thelight from the Bragg gratings.

The light guide may comprise an optical fibre such as a single modeoptical fibre in which the Bragg gratings may have been written. Asoptical fibres are known to cause very little signal loss per length,the apparatus can have a relatively long optical fibre lead and anoptical analyser for analysing the response from the or each Bragggrating may be remote from the or each Bragg grating, such as 1 m, 10 m,1 km or 100 km remote from the or each Bragg grating.

The invention will be more fully understood from the followingdescription of specific embodiments of the invention. The description isprovided with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 (a) and (b) shows a system for distributed pressure sensingaccording to a specific embodiment of the present invention,

FIGS. 2 (a) and (b) show components of an apparatus for distributedpressure sensing according to an embodiment of the present invention andFIG. 2 (c) shows an alternative component of the apparatus for pressuresensing,

FIG. 3 shows a plot of Bragg grating responses as a function oftemperature,

FIGS. 4 (a) and (b) show components of an apparatus for distributedpressure sensing according to a specific embodiment of the presentinvention,

FIGS. 5 (a) and (b) show components of an apparatus for distributedpressure sensing according to a specific embodiment of the presentinvention,

FIG. 6 shows an apparatus for distributed pressure sensing according toanother specific embodiment of the present invention and

FIG. 7 shows an apparatus for distributed pressure sensing according toyet another specific embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring initially to FIG. 1 (a), a system for distributed pressuremeasurement according to a specific embodiment of the present inventionis now described. The system 100 comprises a light source 102 which inthis embodiment is a broadband light source commonly referred to as a“white” light source even though the light that is emitted by the lightsource 102 may have any wavelength range.

The light is directed via optical circulator 104 to an apparatus fordistributed pressure sensing 106. In a variation of this embodiment thecirculator 104 may be replaced by an optical coupler, an opticalsplitter or an optical beam splitter.

The apparatus 106 may comprise a catheter (not shown) for insertion intothe human body. Further, the apparatus 106 typically comprises an X-rayopaque material, such as a metallic material, for locating the apparatus106 in the human body.

The apparatus 106 comprises a series of Bragg gratings 108 which areformed in an optical fibre 110. Each Bragg grating 108 is in thisembodiment positioned in association with an enclosure 112. Eachenclosure 112 has a movable wall portion which is provided in the formof a diaphragm (not shown). In this embodiment, the optical fibre 110 isrigidly connected to end-portions 113 and 115 of a respective enclosure112 so that respective Bragg gratings are positioned between theend-portions 113 and 115. Each Bragg grating 108 is positioned on ornear a respective diaphragm such that a local external pressure changeeffects movement of a respective diaphragm which in turn will apply astrain to a respective Bragg grating 108. The strain on each Bragggrating causes a change of an optical property of the Bragg gratings108, such as a change of an optical path length, which influences anoptical response of the gratings 108 to light reflected from the Bragggrating 108.

As the optical fibre 110 is attached to the end-portions of eachenclosure, the rigid enclosures 112 prevent a strain of one of thesensing regions from being affected by a change in strain at a fibreportion outside that sensing region (for example at another sensingregion). Consequently, it is possible to conduct distributed pressuremeasurements at a plurality of locations and independent from oneanother, even in an environment such as the oesophagus in which an axialstrain is inherently applied to the sensor array.

It will be appreciated, that in alternative embodiments each Bragggrating 108 may be positioned within or below a respective diaphragm.The remaining walls of the enclosure 112 are formed from a rigidmaterial, such as silicon, a plastics or metallic material (for examplestainless steel, invar, tungsten, or kovar), or any other suitable rigidmaterial. In this embodiment the apparatus 106 comprises a series ofthree Bragg gratings 108. In alternative embodiments the apparatus 106may comprise two or more than three Bragg gratings at any fixed orvariable pitch.

In the embodiment described above and illustrated in FIG. 1 each Bragggrating 108 of the series has a slightly different refractive indexvariation so that each Bragg grating 108 has an optical response thathas a slightly different spectral distribution. The light that isproduced by light source 102 and that is directed to the Bragg gratings108 therefore causes three unique responses from the Bragg gratings 108which are directed via the optical circulator 104 to optical analyser114 for optical analysis. Such a procedure is commonly referred to aswavelength division multiplexing (WDM). The Bragg gratings may alsoeffect optical responses which overlap in wavelength or frequency spaceas long as sufficient information is known about each Bragg grating toallow the signals to be successfully deconvolved.

As in this embodiment each Bragg grating 108 causes a differentresponse, it is possible to associate a particular response with aposition along the apparatus 106 to perform distributed pressuremeasurements and detect relative pressure differences between thepositions of the Bragg gratings 108 in the series. The combined responsefrom the Bragg gratings is wavelength division multiplexed and theoptical analyser 114 uses known wavelength division de-multiplexingtechniques to identify the responses from the respective gratingpositions. Suitable software routines are used to determine a pressureor pressure distribution from the optical responses received from theBragg gratings. Pressure measurements typically include calibrating theapparatus.

In a variation of this embodiment at least some of the Bragg gratings108 may be identical and consequently, if the strain conditions are thesame, their optical response will also be the same. In this case apulsed light source may be used to guide light to the Bragg gratings andthe positions of the Bragg gratings may be estimated from a time atwhich the responses are received by the optical analyser 114.

In one particular example the reflectivity of each Bragg grating 108 ischosen so that each response has, at the location of the opticalanalyser 114, approximately the same intensity.

It will be appreciated that in a further variation of this embodimentthe apparatus may be arranged so that responses from respective Bragggratings can be analysed by receiving light that is transmitted throughthe Bragg gratings 108. For example, in this case the apparatus 106typically is arranged so that light is guided from the light source 102through the Bragg gratings 108 and then directly to the optical analyser114.

In this embodiment each Bragg grating 108 is written into an opticalfibre and spliced between fibre portions 110. It will be appreciated,that in alternative embodiments the Bragg gratings 108 and the fibreportions 110 may be integrally formed from one optical fibre. The sameoptical fibre may be used for writing respective refractive indexvariations for each grating so that spaced apart Bragg gratings areformed separated by fibre portions. In this embodiment the enclosures112 comprise a rigid material while the fibre, portions 110 arerelatively flexible. Consequently the apparatus 106 is an articulateddevice. FIG. 1 (b) shows the system for pressure sensing 100 also shownin FIG. 1 (a), but, between the enclosures 112 the optical fibre 110 isbent.

FIGS. 2 (a) and (b) show schematically a unit of an apparatus fordistributed pressure sensing in more detail. The apparatus fordistributed pressure sensing comprises a series of the units 120. Eachunit 120 comprises an optical fibre 122, a Bragg grating 124 and anenclosure 126 which includes a body 128, a diaphragm 130 and an anvil132. The optical fibre 122 is attached to the body 128, which iscomposed of a rigid material, at attachment regions 127 and 129 so thateach Bragg grating 124 is positioned between respective attachmentregions 127 and 129. In this embodiment attachment is effected using asuitable glue but a person skilled in the art will appreciate thatvarious other means may be used to secure the Bragg grating 124 to thebody 128. The enclosure 126 encloses a volume 134 and is arranged sothat a change in external pressure will change the enclosed volume 134by deflecting the diaphragm 130 and the anvil 132. This results in aforce on the Bragg grating 124 between the attachment regions and fromone side which increases a distortion of the Bragg grating 124. In thisembodiment the Bragg grating 124 is distorted into the enclosed volume134. This arrangement prevents an axial force acting on fibre 122external to the enclosure and the attachment regions 127 and 129 fromaffecting the optical response of the Bragg grating 124.

FIG. 2 (c) shows an enclosure 133 which is a variation of the enclosure126 shown in FIG. 2 (a). The enclosure 133 has two portions 135 and 137for securely fixing a Bragg grating and two recesses 139 and 141 forretaining the optical fibre containing the Bragg grating in a flexiblemanner. The flexible coupling portions reduce bending forces at theportions 135 and 137 on the Bragg grating.

It is to be appreciated that the apparatus shown in FIG. 2 has only oneof many possible designs. For example, each unit of the apparatus fordistributed pressure sensing may not necessarily have an anvil but theBragg grating may be mechanically distorted into the enclosed volumewithout an anvil and in contact with the diaphragm.

FIGS. 4 (a) and 4 (b) show further examples of a unit of an apparatusfor distributed pressure sensing according to another embodiment of thepresent invention. The apparatus for distributed pressure sensingcomprises a series of the units which are optically coupled for exampleby an optical fibre. Each unit 200 comprises a Bragg grating 202 and abody 204. The Bragg grating 202 is formed in an optical fibre thatcomprises a core/cladding region 205 and a protective coating 206. Theprotective coating 206 has been stripped away in the area of the Bragggrating 202. The core/cladding region is attached to the body 204. Inthis embodiment the core/cladding region 205 is glued to the body 204 atregions 210 and 212. For example, the body may be formed from silicon, aplastics or metallic material, or any other suitable rigid material.

FIG. 4 (b) shows a unit 220, a variation of the unit 200, with adiaphragm 214 applied to it. For example, the diaphragm 214 may be acold or hot shrink tube which is located over the Bragg grating 202 andover the body 204 or an elastic material that stretches around the body204. As the body 204 has a recess 216, an enclosed pressure sensitivevolume is formed at the recess 216 and below the diaphragm 214. Thediaphragm 214 is composed of a flexible material such as a rubber ornylon material, a flexible metal foil or silicone foil. Similar to theembodiment shown in FIG. 2, the Bragg grating 202 is slightly distortedinto the enclosed volume in the recess 216 (the distortion is indicatedin FIG. 4 (b) and not shown in FIG. 4( a)).

FIG. 3 shows plots of Bragg grating responses as a function oftemperature. Plot 140 shows the response of a grating of an embodimentof an apparatus for pressure sensing which is schematically shown inFIG. 4( b). In this example, the enclosure 204 is formed from stainlesssteel and the diaphragm is formed from polyolefin heat shrink. FIG. 3shows also a plot 142 for a typical Bragg grating that is not coupled toan enclosure and to a diaphragm and a plot 144 for a Bragg gratingbonded to a stainless steel substrate and enclosed by TEFLON(poly(tetrafluoroethylene) or poly(tetrafluoroethene) (PTFE)) tape(3M#60 PTFE tape).

An optical response of the Bragg grating typically has a lineardependency on the temperature and on axial strain, but the strain on thefibre in the enclosures described herein typically has a quadraticdependency on the temperature. Consequently, if a Bragg grating isarranged so that a change in temperature of the enclosure also causes achange in strain, the optical response of the Bragg grating will have acombined quadratic and linear dependency on the temperature.

In this example the distortion of the Bragg grating 202 and the designof the enclosure 204 are selected so that the optical response of theBragg grating does not change by more than approximately 0.001 nm if thetemperature changes by ±1 degree from the normal operating temperatureof the apparatus centred at approximately 77° C.

In the example of the units 200 and 220 shown in FIGS. 4( a) and (b) thedistortion of the Bragg grating 202 causes a tensile strain of the Bragggrating 202. If the ambient temperature now increases from the normaloperation temperature, a number of physical effects may take place. Theoptical period of the Bragg grating 202 will typically increase and theenclosed volume will tend to expand. Further, the diaphragm material,which typically is positioned so that the distortion of the Bragggrating is increased at a normal operating temperature, will tend toexpand and/or the Young's modulus of the diaphragm material may decreasewhich in turn causes a decrease of the distorting force on the Bragggrating 202 and thereby counteracts the increase of the optical period.Hence, it is possible to influence the temperature dependency of opticalresponses by selecting materials having selected thermal behaviour.

Since typically all of the above physical processes influence thegrating response as a function of temperature, it is possible to selectan enclosure design and a Bragg grating distortion so that the valley ofthe plot 140 can be shifted to a wide range of temperatures. Further, itwould be possible to design the apparatus so that the plot 140 wouldhave more than one valley and/or peak and hence provide an extendedrange over which acceptable athermal behaviour is achieved.

In this example the valley in plot 140 is positioned at approximately77° C., but a person skilled in the art will appreciate that in avariation of this embodiment the apparatus may be designed so that thevalley is positioned at approximately 37° C. (or normal bodytemperature) which would then be the normal operating temperature.

FIGS. 5 (a) and 5 (b) shows units 300 and 330 according to furtherembodiments of the present invention. The apparatus for distributedpressure sensing may comprise a series of the units 300 or 330 which areoptically coupled for example by an optical fibre. Both the units 300and 330 comprise the Bragg grating 202, the fibre core/cladding 205 andthe protective coatings 206. The unit 300 comprises a body 302 to whichthe core/cladding region 205 is glued at regions 304 and 306. In thisembodiment the body 302 has a substantially rectangular cross sectionalarea and may be formed from silicon or any other suitable rigidmaterial.

The unit 300 further comprises a flexible cover not shown), such as adiaphragm, which is positioned over the Bragg grating 202 and enclosesrecess 308 of the rigid structure 302. Alternatively, the cover may bepositioned below the Bragg grating 202 and may cover the recess 308 sothat an enclosed internal volume is formed below the Bragg grating 202.In this case the Bragg grating 202 typically is connected to the coverso that a movement of the cover causes a strain to the Bragg grating andconsequently a pressure change can be sensed.

The unit 330 shown in FIG. 5 (b) comprises a rigid casing 332 which hasa flexible cover 334. The casing 332 is hollow and the flexible cover334 closes the casing 332 to form a hollow internal volume below theBragg grating 202. As in the previous example, the flexible cover may bea diaphragm. The Bragg grating 302 is attached to the flexible cover sothat a movement of the flexible cover will cause a strain in the Bragggrating. The casing 332 typically is composed of a silicon material orof any other suitable rigid material. The flexible cover 334 typicallyis a thin layer that provides sufficient flexibility and is composed ofsilicon, silicone, another polymeric material or a suitable metallicmaterial.

The examples of the units for pressure sensing shown in FIGS. 2, 4 and 5are suitable for asymmetric pressure sensing. For example, a pressureincrease located only at the rigid portions of the casings 304, 303 or332 will typically not cause a strain to the Bragg gratings 202. FIG. 6shows a unit for pressure sensing according to a further embodiment ofthe present invention which can be used for more symmetric pressuremeasurements. Again, the apparatus for distributed pressure sensing maycomprise a series of the units 400 which are optically coupled forexample by an optical fibre.

The unit 400 shown in FIG. 6 comprises a rigid structure 402 havingrigid upper and lower portions 404 and 406 and a rigid support portion408 connecting the upper and lower portions 404 and 406. The rigidsupport portion is surrounded by a diaphragm 410 which is applied to theupper and lower portions 404 and 406 so that an enclosed internal volumeis formed. The apparatus 400 also comprises a Bragg grating 412 and acore/cladding region 414. The core/cladding region 414 is attached tothe upper and lower portions 404 and 406 at positions 418 and 420. Inthis embodiment the core/cladding region is glued at these positions tothe upper and lower portions 404 and 406 respectively, and attached tothe diaphragm 410.

For example, the Bragg grating 412 may be attached to the diaphragm 410using a flexible adhesive. If a pressure in a region adjacent thediaphragm 410 changes, the diaphragm 410 will move which will cause astrain in the Bragg grating 412 and therefore the pressure change can besensed. As the Bragg grating 412 is wound around the diaphragm 410 andthe diaphragm 410 surrounds the support 408 so that internal volume isformed between the support 408 and the diaphragm 410, a pressure changecan be sensed at any position around the diaphragm 410 using the device400. Similar to the embodiments discussed before, the Bragg grating 412is slightly distorted into the enclosed volume (the distortion into theenclosed volume at the normal operating temperature is not shown in FIG.6).

The rigid portions 404, 406 and the support 408 typically are composedof silicon or of any other suitable rigid material including plastics ormetallic materials. The diaphragm 410 typically is a thin layer having athickness of the order of 0.1 mm being composed of silicone, anotherpolymeric material or a metallic material.

The hereinbefore-described apparatus for pressure sensing according todifferent embodiments of the present invention comprises an enclosurethat defines an enclosed space and of which the diaphragm forms a part.In a variation of these embodiments, the apparatus for pressure sensingmay not comprise such an enclosure and FIG. 7 shows an example of suchan alternative design. FIG. 7 shows an apparatus for pressure sensing500 having an optical fibre with the Bragg grating 202 and which isattached to rigid member 504 at attachment regions 506 and 508.Diaphragm 510 distorts the Bragg grating at a normal operatingtemperature and separates a first region having a first pressure P₁ froma second region having a second pressure P₂. A relative change in thepressures P₁ and P₂ will move the diaphragm 510 and thereby cause achange in a force on the Bragg grating 202. As in the above-describedembodiments, the diaphragm 510 and the Bragg grating 202 are positionedso that a temperature related change in optical response of the Bragggrating 202 is reduced by a temperature related change in the force onthe Bragg grating. For example, the apparatus for pressure sensing 500may be positioned across a conduit, such as a tube, for measuring apressure caused by a flow of a fluid.

Although the invention has been described with reference to particularexamples, it will be appreciated by those skilled in the art that theinvention may be embodied in many other forms. For example, theapparatus for pressure sensing may comprise Bragg gratings that arepositioned within the diaphragms. Further, the enclosures may have anysuitable shape with which an enclosed internal volume can be formed whena diaphragm is applied to it. It is also to be appreciated that inalternative embodiments the enclosures may not comprise rigid membersbut rigid members may be positioned between adjacent Bragg gratings ofthe series of Bragg gratings.

The invention claimed is:
 1. An apparatus for distributed pressuresensing, the apparatus comprising: a light guide incorporating a seriesof Bragg gratings at a series of sensing regions; a plurality ofmoveable wall portions having opposite first and second sides, eachmoveable wall portion being positioned so that a change in pressure atone of the sides relative to a pressure at the other side will move themoveable wall portion, the moveable wall portions being coupled to theBragg gratings so that the movement of each of the moveable wallportions causes a force on at least a corresponding one of the Bragggratings, resulting in a change in strain of that Bragg grating; and atleast one rigid member that is rigidly attached to attachment regions ofthe light guide, the attachment regions including at least one pair ofadjacent first and second attachment regions, the first and secondattachment regions separated by at least one of the sensing regions,wherein the at least one rigid member is arranged so that a change instrain of the light guide outside the sensing regions does not produce achange in a strain in the sensing regions, does not produce a change inan optical path length of the Bragg gratings, and does not change anoptical response of the apparatus.
 2. The apparatus of claim 1, furthercomprising a plurality of enclosures, each enclosure enclosing a spaceand comprising a respective rigid member and a respective moveable wallportion coupled to a respective Bragg grating.
 3. The apparatus of claim2 wherein each enclosure is attached to the light guide at theattachment regions so that a respective sensing region of a respectiveBragg grating is positioned between the attachment regions and theenclosure comprises the rigid member and prevents a strain of thatsensing region from being directly influenced by a change in strain ofthe light guide outside that sensing region.
 4. The apparatus of claim 3wherein the optical light guide is flexible at regions between adjacentenclosures so that the apparatus is articulated.
 5. The apparatus ofclaim 2 having a normal operating temperature and pressure range atwhich the Bragg gratings are distorted into respective enclosed spaces.6. The apparatus of claim 1 wherein each Bragg grating of the series isarranged to give a different optical response so that light reflectedfrom each Bragg grating is wavelength division multiplexed.
 7. Theapparatus of claim 1 wherein at least some of the Bragg gratings areidentical and give the same response if the strain conditions are thesame.
 8. The apparatus of claim 1 wherein the light guide comprises oneoptical fibre.
 9. The apparatus of claim 1 wherein the optical fibrecomprises portions that are spliced together.
 10. The apparatus of claim1 being arranged so that each force is applied from one side of arespective Bragg grating.
 11. The apparatus of claim 1 being arranged sothat each force is applied in a direction that is perpendicular to anaxis of a respective Bragg grating.
 12. The apparatus of claim 1 whereineach moveable wall portion is a diaphragm.
 13. The apparatus of claim 12wherein each Bragg grating is positioned on one of the diaphragms andoutside the enclosure.
 14. The apparatus of claim 12 wherein each Bragggrating is positioned within the diaphragm.
 15. The apparatus of claim12, wherein each Bragg grating is positioned on the diaphragm and insidethe enclosure.
 16. The apparatus of claim 1 comprising an externalcatheter.
 17. The apparatus of claim 1 comprising an X-ray opaquematerial.
 18. A method of measuring a pressure in an in-vivoenvironment, the method comprising: inserting an apparatus for pressuresensing into a body, the apparatus comprising a light guide and a seriesof Bragg gratings incorporated into the light guide, exposing theapparatus to a pressure in the in-vivo environment so that the pressurecauses a force on the Bragg grating which displaces the entire Bragggrating in a transversal direction and changes a strain of the Bragggrating and thereby changes an optical period of the Bragg grating,guiding light to and from the series of the Bragg gratings and receivinga response from the series of the Bragg gratings.
 19. The method ofclaim 18 comprising the step of converting optical data into pressuredata.
 20. A method of measuring a muscular pressure in an in-vivoenvironment comprising the method as claimed in claim
 18. 21. A methodof measuring a muscular pressure in the oesophagus comprising the methodas claimed in claim 18.